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Recent advances in the water–energy landscape hinge upon our improved understanding of the complex morphology of materials involved in water treatment and energy production. Due to their versatility and tunability for applications ranging from drug delivery to fuel cells, polymeric systems will play a crucial role in shaping the future of water–energy nexus applications. Electron tomography (ET) stands as a transformative approach for elucidating the intricate structures inherent to polymers, offering unparalleled insights into their nanoscale architectures and functional properties in three dimensions. In particular, the various morphological and chemical characteristics of polymer membranes provide opportunities for perturbations to standard ET for the study of these systems. We discuss the applications of transmission electron microscopy in establishing structure–function relationships in polymeric membranes with an emphasis on traditional ET and cryogenic ET (cryo-ET). The synergy between ET and cryo-ET to unravel structural complexities and dynamic behaviors of polymer membranes holds immense potential in driving progress and innovation across frontiers related to water–energy nexus applications. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering , Volume 15 is June 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates. 

Abstract Pathogen contamination of water has a massive impact on global human health. In particular, viruses pose unique challenges to water treatment techniques due to their small size and presence in water as both individual virions and when absorbed onto larger particles. Low-energy water treatment processes such as media filtration are not capable of completely removing viruses owing to their small size. Hence, less sustainable processes with high chemical or energy consumption such as chemical disinfection, ultraviolet irradiation, and membrane filtration are usually required. To overcome high energy and/or chemical requirements for virus treatment, designs for sustainable fiber filters fabricated from minimally processed natural materials for efficient virus (MS2) and bacteria (E. coli) removal are presented in this work. These filters were created by functionalizing readily accessible natural fibers including cotton, silk, and flax with a simple aqueous extract containing cationic proteins fromMoringa oleiferaseeds. The proposed filters offer a comprehensive low cost, low energy, and low environmental impact solution for pathogen removal from water with removals of >7log₁₀(99.99999%) for viruses and bacteria. 

Biological membranes can achieve remarkably high permeabilities, while maintaining ideal selectivities, by relying on well-defined internal nanoscale structures in the form of membrane proteins. Here, we apply such design strategies to desalination membranes. A series of polyamide desalination membranes—which were synthesized in an industrial-scale manufacturing line and varied in processing conditions but retained similar chemical compositions—show increasing water permeability and active layer thickness with constant sodium chloride selectivity. Transmission electron microscopy measurements enabled us to determine nanoscale three-dimensional polyamide density maps and predict water permeability with zero adjustable parameters. Density fluctuations are detrimental to water transport, which makes systematic control over nanoscale polyamide inhomogeneity a key route to maximizing water permeability without sacrificing salt selectivity in desalination membranes.